Hospital outbreak of methicillin-resistant Staphylococcus aureus caused by a new, possibly hyperepidemic variant from a previously sporadic strain.

Objective: To describe an ongoing outbreak that tripled the annual detection of methicillin-resistant Staphylococcus aureus (MRSA) carriage in a tertiary care hospital. Methods: Active surveillance of MRSA is performed since 20 years in our hospital. Our protocol includes screening of patients transferred from high-incidence health-care institutions or countries, roommates of new MRSA cases, and wards where _2 patients acquired MRSA during the same week. Contact precautions are used for known carriers. PFGE was used for molecular typing until 2004, and was then replaced by Double-Locus Sequence Typing (DLST). Results: A median yearly incidence of 173 new carriers of MRSA was observed from 2002 to 2007. Since September 2008, an increasing number of new cases were observed, mainly as successive clusters limited to distinct wards, reaching a total of 398 until October 2009. The yearly incidence of new cases rose to 275 in 2008 and 613 in 2009. 60% of the cases were due to one strain: DLST 4−4, ST 228, SCCmecI. The incidence of new cases due to the previously predominant strains remained unchanged. The epidemic strain corresponded to a new variant of a clone responsible for a previous outbreak in 2001, and only sporadically isolated (mean of 20 cases/year) since then. A case- control study documented a significant association between acquisition of the epidemic strain and a stay in intensive and intermediary care units, a highest number of internal transfers, but did not identify a point source of transmission. Infection control practices and antibiotic policy had remained unchanged for several years. Compliance with handhygiene as monitored yearly was on the rise. Screening of 313 healthcare workers only found one carrier of the epidemic strain lately in the outbreak. Additional infection control measures were enforced, including screening at ICU admission and discharge with PCR-based rapid test, routine screening for all patients leaving epidemic wards, introduction of PCR-based rapid test for contact tracing, additional working forces for environmental disinfection, and hospital-wide education of healthcare workers. However, the outbreak was still ongoing after 5 months. Conclusions: Factors linked to the dissemination of this new variant in our institution remain undetermined. This unresolved outbreak suggests that this new variant acquired hyperepidemic properties, which calls for further investigations.